The present invention relates to the transfer and storage of cryogenic fluids, and, more particularly, to cryogenic fluid transfer lines and storage vessels utilizing novel support means to increase their operating efficiency by reducing heat transfer into such systems.
The increasing use of cryogenic fluids in various scientific, medical, and technical fields has created the need for transfer lines and storage vessels with excellent thermal isolation characteristics for the efficient storage and transfer of these fluids. Because cryogenic fluids must be maintained at temperatures well below ambient, heat transfer, be it by radiation, conduction, or convection, into a cryogenic transfer line or storage system can result in loss of refrigeration which drives up operating costs and slows down fluid flow within the transfer line. The loss of refrigeration from heat transfer is amplified greatly by the power needed to recover such losses. For example, even with a cryogen recovery system, a heat transfer of 1 watt into a transfer line at 4K will take approximately 400 watts of electrical power to recover, provided that a very efficient refrigeration system with a Carnot efficiency of 30% is available. When a recovery system is not available, the penalty is even more severe, as the heat transfer is eventually translated into cryogen loss into the atmosphere.
In a typical cryogenic fluid transfer line wherein an inner fluid delivery tube is contained within an outer vacuum tube, or vessel, thermal insulation of the inner delivery tube is obtained by means of a vacuum which surrounds and separates the inner tube from the outer vacuum vessel. A major source of heat transfer into the inner delivery tube arises from the supports or spacers which align and suspend the inner tube within the outer vacuum vessel. Although there are various known configurations for the supports and spacers, the conventional design calls for thin circular disks, made from composite materials to be placed within the vacuum vessel to support and align the inner tube as it travels longitudinally in the vacuum vessel. Such support disks are then positioned at designated intervals along the transfer line to continuously align and support the inner delivery tube. However, such supports or spacers introduces considerable heat transfer, via conduction, into the inner cryogenic delivery tube.
The problem becomes even more acute when a plurality of delivery tubes of differing temperatures are enclosed within a common vacuum vessel and supported by a common disk. As is the case at the Superconducting Super Collider Laboratory and other facilities, various superconducting applications require that the cryogenic transfer line transport cryogen of various temperatures. In such applications, because a plurality of delivery tubes at differing temperatures share the same supporting disk, the inner delivery tubes are not only in thermal contact with the vacuum vessel but also with each other. Since the support disk tends to maintain a temperature warmer than the coolest delivery tube, heat conduction occurs directly from the support disk to the coldest delivery tube to create a very short heat conduction path. Accordingly steep temperature gradients are created within the support disk which intensify heat conduction into the transfer line.
Additionally, in applications utilizing a plurality of delivery tubes at different temperatures, an added difficulty arises from the thermal stress related to the expansion and contraction of the tubes. Since the support disk and the multiple delivery tubes have differing temperatures, their expansion/contraction rates vary and create thermally induced stresses between the various members in contact. Accordingly, the support disk, which experiences both longitudinal and radial stress from the delivery tubes, must be made thicker to absorb such stress, which, in turn, further aggravates the conductive heat transfer into the system.
Similarly, these same heat transfer and thermal stress considerations also apply to a standard cryogenic fluid storage vessel wherein an inner storage tank is suspended within an evacuated outer shell by various supporting means. Likewise, these problems become amplified when a storage vessel comprises a plurality of such inner tanks containing cryogen at different temperatures.
In view of the foregoing, the general object of this invention is to provide an apparatus for supporting an inner containment system within an outer enclosure to retard heat transfer into the inner containment system.
Another object of this invention is to provide an efficient cryogenic fluid transfer line utilizing novel support systems to retard heat transfer into the transfer line.
Yet another object of this invention is to provide a cryogenic fluid transfer line utilizing novel support systems to minimize thermally induced stress within the line.
A further object of this invention is to provide a storage vessel for cryogenic fluids utilizing a novel support system to reduce heat conduction into the vessel.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.